For any given radiation treatment procedure, a common goal is to deliver an adequate, therapeutically effective dosage of radiation to the tumor while minimizing potentially damaging dosage exposure to nearby critical organs and other tissues (termed organs at risk or OAR). Intensity-modulated radiotherapy (IMRT) is a popular new technology to customize radiation treatment in accordance with this goal for each patient. The most widely available apparatus for delivering IMRT procedures for cancer patients is a medical linear accelerator (LINAC). LINAC-based IMRT treatment apparatuses are described in, for example, U.S. Pat. Nos. 5,663,999; 6,134,296; 6,240,161; 6,240,162; 6,330,300; 6,335,961; 6,349,129; 6,353,655; 6,449,335; 6,473,490; and 6,477,229, the respective contents of which are incorporated herein in their entireties.
Compared to conventional LINAC-based radiotherapy treatments, an IMRT treatment delivers a variable intensity distribution, or intensity map, within a treatment portal. The intensity maps are normally designed by a dose optimization algorithm, whose objective is defined by the radiation oncologist for the patient to meet a set of dose distribution criteria. The criteria often involve the tumor volume and structures of nearby organs at risk (OAR). The intensity maps, sometimes along with other treatment parameters, are used as variables by the dose optimization algorithm to design the optimal treatment. The dose optimization algorithms generally are an integral part of treatment planning systems, and can be implemented in treatment planning software (TPS).
One difference in dose optimization algorithms is the resolution of the intensity map they generate, i.e., whether the intensity map is continuous or discrete. When the limitations of treatment delivery technique are taken into consideration in the dose optimization process, such as occurs in conventional multi-leaf collimator (MLC) treatment delivery techniques (described hereinbelow), the resulting intensity maps are discrete. When these limitations are not considered, the optimization can often produce continuous intensity maps that represent the ideal IMRT treatment. It has been demonstrated that the dosimetric quality of an actual IMRT treatment can be considerably affected by the resolution of the delivery technique compared to the ideal treatment. See Chang et al., Intensity modulation delivery techniques: “Step & shoot” MLC auto-sequence versus the use of a modulator, Med. Phys. 27, 948 (May 2000), the content of which is incorporated herein in its entirety; and Potter et al., A quality and efficiency analysis of the IMFAST™ segmentation algorithm in head and neck “step & shoot IMRT treatments, Med. Phys. 29 (3), 275-283 (March 2002), the content of which is incorporated herein in its entirety. See also Chang et al., Dose optimization via index-dose gradient minimization, Med. Phys. 29, 1130 (June 2002), the content of which is incorporated herein in its entirety.
Virtually all treatment planning systems, commercial or in-house, have implemented, or are in the process of doing so, dose optimization algorithms to design IMRT treatments. The three-dimensional TPS at the University of North Carolina at Chapel Hill, which is termed PlanUNC (PLUNC), has implemented the index-dose gradient minimization optimization algorithm developed by Chang et al., Dose gradient optimization, Med. Phys. 23, 1072 (1996), the content of which is incorporated herein in its entirety.
Once the dose optimization algorithm of PLUNC generates the ideal intensity maps, an example of which is shown in FIG. 1, further processing of this and other treatment information in preparation for actual delivery to the patient will depend on the particular IMRT delivery technique to be utilized. LINAC-based IMRT delivery techniques can generally be categorized into two types: compensator- or modulator-based and multi-leaf collimator- or MLC-based IMRT methods. Compensator-based techniques employ a compensator, or intensity modulator. The compensator is a physical beam attenuation device placed in the beam path of the radiation treatment apparatus during treatment. The compensator is customized to selectively attenuate the photon fluence and thus deliver the intended fluence distribution to the patient. The compensator-based technique delivers each IM field statically, i.e., the modulation does not vary with time during treatment. The modulation in the MLC-based IMRT techniques, however, varies in both time and space. Multi-leaf collimator (MLC) techniques utilize a built-in or added-on device of modern medical accelerators to deliver the intensity modulation. As appreciated by persons skilled in the art, an MLC device typically comprises at least two transversely opposing sets of radiation-blocking leaves that define the radiation field capture on a patient (see, e.g., leaves L in FIG. 11). The leaves of the respective leaf sets of the MLC device are movable toward and away from each other in order to define an opening or port through which the radiation beam is delivered to the patient.
The MLC techniques can be further categorized into two types: the “dynamic” or “sliding window MLC technique and the step-and-shoot” or “stop-and-shoot” technique. The dynamic MLC technique delivers an intensity modulated photon field by moving the collimator leaves during irradiation. The “step-and-shoot” MLC technique delivers an intensity modulated photon field via a sequence of static MLC ports. Each MLC configuration or the port opening defined thereby is termed a “segment” or “segment-field”. The intended intensity map is delivered to the patient cumulatively in an MLC-based delivery technique. That is, the treatment of the IM field is delivered through the cumulative number of automatically sequenced segments or segment-fields.
The “step-and-shoot” MLC technique requires an MLC-IMRT segmentation algorithm to design the MLC leaf configuration and monitor units (MU's) for each segment based on the intended intensity map.
Most MLC sequence segmentation software products available now require discrete intensity maps to generate the segment-fields of “step & shoot” MLC-IMRT treatment. An example of a discrete intensity map is shown in FIG. 2. FIG. 3 illustrates a sequence of eight segments generated for the 5-IM-level map of FIG. 2, for the left lateral field of a sinus treatment. In the segment with the largest area, or base segment, a significant area of the left side can be seen to be blocked by the MLC leaves to minimize the radiation dosage to the eyes of the patient. MLC sequence optimization software systems, such as the IMFAST™ system commercially available from Siemens Medical Systems, Inc., Concord, Calif., take into account the mechanical constraints of the MLC leaf positioning, the “tongue and groove” effect, a simple photon source model fitted to the accelerator beam data, and other relevant parameters in the MLC sequence optimization. MLC sequence optimization software systems such as IMFAST™ make available several different sequence optimization methods, all of which strive to minimize the number of MLC segments and treatment delivery time, and strive to deliver the IM field as close to the inputted discrete IM map as possible. All segments of the same IM field can be grouped together and delivered automatically with a keyboard operation similar to the delivery of a single field.
The intensity map resolution yielded by the “step and shoot” MLC delivery technique has a significant effect on the efficiency and the quality of the IMRT treatment. The resolution of the intensity map can be considered as being represented by two resolutions. The first resolution is that of the intensity levels, i.e., the number of intensity steps that are used to synthesize the intensity map. The second is the spatial resolution of the intensity map. The intensity maps employed by the IMFAST™ system have a user-defined number of discrete intensity levels and a finite spatial resolution related to the width of each MLC leaf. In one study, it was shown that the quality of a dose-optimized IMRT treatment is directly related to the resolution of the intensity maps delivered by “step and shoot” treatments. See Chang et al., Intensity modulation delivery techniques: “Step & shoot” MLC auto-sequence versus the use of a modulator, Med. Phys. 27, 948 (May 2000). This study showed that when the smooth intensity maps originally generated by a dose optimization algorithm are truncated into corresponding discrete maps, a substantial deterioration in dose distributions can result-not only in the target but also in the nearby critical/normal structures. The treatment quality sometimes can be improved by increasing the intensity level resolution of the intensity map, but at the cost of increasing the number of segments required and thus reducing the efficiency of the treatment.
A recent technique has been developed to refine the intensity map resolution by effectively improving the spatial resolution of the technique from 1-cm×1-cm to 5 mm×5 mm by modification treatment hardware (smaller and more MLC leaves and treatment table shifts during treatment). However, this hardware solution has the drawback of increasing the mechanical complexity and thus the cost of the device.
Some of the available segmentation algorithms are designed to find the set of MLC segment-fields (and their corresponding MU values) for cumulatively delivering an intensity map that matches the input map as close as possible and requires the shortest treatment time. Unfortunately, the discreteness used in existing MLC-based IMRT dose optimization and segmentation algorithms is considerably stricter than the intrinsic limitations of the existing MLC delivery hardware. For instance, although the width of the MLC leaves is fixed, the position of each MLC leaf along its travel direction is continuously adjustable within its range of motion. In addition, the angular orientation of the MLC leaves, or the collimator angle, is continuously adjustable within its range of motion. It is proposed herein that these variables are degrees of freedom in the MLC segmentation. It is further proposed herein that, once properly utilized, these degrees of freedom can improve the segmental MLC technique to deliver a much finer-resolution intensity modulation with the same or better treatment efficiency. The angle of the collimator can have a significant influence of the discrepancy between the discrete “skyscraper” IM map and its corresponding original smooth map. The effect of the collimator angle is similar to that in conforming an MLC opening to a given treatment portal defined by a conventional block. An optimal collimator angle can minimize the jaggedness of the edge or contour of the field defined by the MLC opening. An optimal collimator angle can reduce the difference between the discrete IM map and its original smooth map. Therefore, it is proposed herein that the orientation of the MLC leaves or collimator angle should be considered as a variable in the MLC-IM treatment delivery optimization process.
A primary advantage of the MLC-IMRT delivery techniques over the modulator delivery techniques is treatment delivery automation. The compensator-based technique, however, has the advantage of high resolution of intensity modulation. In a comparative study of “step and shoot” MLC-IM and modulator-IM treatment techniques for two clinical cases, a three-field sinus tumor treatment and a six-field nasopharynx tumor treatment, the dose optimization quality of each treatment technique was judged by how well the defined optimization goal was reached for each case. See Chang et al., Intensity modulation delivery techniques: “Step & shoot” MLC auto-sequence versus the use of a modulator, Med. Phys. 27, 948 (May 2000). It was found that target dose uniformity initially improved quickly as the IM level increased to 5, then started to approach saturation when the MLC technique was performed. A linear proportionality was found between the number of IM levels used and the number of MLC segments required. For both clinical cases, the proportionality was between one to two segments per IM level per field. Both clinical cases suggested that an IM level of 5 offered a good compromise between the dose optimization quality and treatment irradiation time. In the absence of both space and intensity discreteness intrinsic to the MLC technique, the modulator-based technique produced greater tumor dose uniformity and normal structure sparing.
The segmental MLC technique also suffers from longer treatment irradiation time. Treatment irradiation time can be defined as the time elapsed between the initiation of a treatment on the console of a medical LINAC-based apparatus, such as by pushing a “RAD ON” button, to the completion of the irradiation. In addition to this treatment time, the compensator-based techniques and other conventional techniques such as wedge-based techniques require time for therapists to enter the treatment room to exchange the modulator or wedge between the application of different treatment fields. In order to render a meaningful inter-comparison of treatment irradiation time among the MLC-IM technique, the compensator-IM technique, and the wedge-based technique, an equivalent treatment irradiation time can be defined as the treatment irradiation time defined above plus the beam modifier exchange time when applicable. In the case of the MLC-IM technique, it has been found that the equivalent treatment irradiation time depends on the IM resolution or the IM level. Better dose optimization quality requires more IM levels, which in turn requires more treatment irradiation time. It has been further found that the modulator-IM and the wedge-based treatment technique requires substantially less treatment irradiation time. For the specific cases studied, the treatment time required by the MLC-IM technique was 100-400% longer than the conventional techniques. See Chang et al., Intensity modulation delivery techniques: “Step & shoot” MLC auto-sequence versus the use of a modulator, Med. Phys. 27, 948 (May 2000).
Therefore, in the case of segmental MLC-IMRT techniques, there is much room for improvement regarding intensity modulation quality and treatment delivery efficiency.
Treatment automation and intensity modulation resolution are both important considerations in IMRT treatment delivery. It would therefore be advantageous to provide a segmentation method that combines the strengths of both general types of delivery techniques, i.e., the treatment automation afforded by segmental MLC-IMRT techniques and the high-resolution intensity modulation afforded by compensator-IMRT techniques. It would also be advantageous to provide a segmentation method that takes full advantage of the degrees of freedom afforded by radiation treatment apparatuses equipped with MLC functionality. It would further be advantageous to provide a segmental MLC-IMRT technique that reduces treatment time.